Transcript Emery Berger Operating Systems CMPSCI 377 Lecture 17: File Systems II

Operating Systems
CMPSCI 377
Lecture 17: File Systems II
Emery Berger
University of Massachusetts, Amherst
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
Outline
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File Systems Implementation
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How disks work
How to organize data (files) on disks
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Data structures
Placement of files on disk
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Disk Surface Layout
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Tracks: concentric rings on disk
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bits laid out serially on tracks
Tracks split into sectors or blocks
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Minimum unit of transfer from disk
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Disk Pack: Multiple Disks
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Disks organized in disk
pack = stack of platters
Use both sides of platters
Two read-write heads at
end of each arm
Cylinders = matching
sectors on each surface
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Cost of Disk Operations
In addition to CPU time to start disk operation:
 Latency: time to initiate disk transfer
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Seek time: time to position head over correct
cylinder
Rotational time: time for correct sector to
rotate under disk head
Bandwidth: rate of I/O transfer of sectors
once initiated
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Outline

File Systems Implementation


How disks work
How to organize data (files) on disks
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
Data structures
Placement of files on disk
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
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File Organization on Disk
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File system maps file blocks to disk location
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(file 0, block 0) = (platter 0, cylinder 0, sector 0)
Key performance issues:
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Support sequential and random access
What data structure is best for maintaining file
location information?
How do we lay out files on the disk?
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On-Disk Data Structures
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File descriptor: structure used to describe
where file is on disk and attributes
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Most systems fit following profile:
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Must be stored on disks just like files
Most files are small
Most disk space taken up by large files
I/O operations target both small & large
Per-file cost must be low, but large files must
also have good performance
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Contiguous Allocation
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Operating system maintains ordered list of
free disk blocks
OS allocates contiguous chunk of free blocks
when it creates a file
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Only need to store start location & size in file
descriptor
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Contiguous Allocation:
Pros & Cons
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Advantages
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Disadvantages
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Simple
Access time? Number of seeks?
Changing file sizes
Fragmentation? Disk management?
Examples: IBM OS/360, write-only disks,
early PCs
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Linked Files
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Maintain list of all free sectors/blocks
In file descriptor, keep pointer to first sector/block
In each sector, keep pointer to next sector
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Linked Files: Pros & Cons
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Advantages
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Disadvantages:
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Fragmentation?
File size changes?
Efficiently supports which type of access?
Does not support which type of access? Why?
Number of seeks?
Examples: MS-DOS
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Indexed Files
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OS keeps array of block
pointers for each file
User or OS declares maximum
length of file created
OS allocates array to hold
pointers to all blocks when it
creates file
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But allocates blocks only on
demand
OS fills pointers as it allocates
blocks
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Indexed Files: Pros & Cons
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Advantages
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Wastes very little space
Sequential & random access: easy
Disadvantages
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Sets maximum file size
Lots of seeks (why?)
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Multilevel Indexed Files
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Each file descriptor contains
14 block pointers
First 12 pointers point to data
blocks
13th pointer: one indirection
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14th pointer: two indirections
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Points to block of 1024
pointers to 1024 more data
blocks
Points to block of pointers to
indirect blocks
Used in BSD UNIX 4.3
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Multilevel Indexed Files, Zoomed In
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Multilevel Indexed Files:
Pros & Cons
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Advantages
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Disadvantages
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Simple to implement
Supports incremental file growth
Small files?
Indirect access: inefficient for random access to very
large files
Lots of seeks (data not contiguous)
Is file size bounded?
What could OS do to get more contiguous memory?
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Free-Space Management: Bitmaps
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Need free-space list to keep track of which
disk blocks are free
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One approach: bitmap
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Akin to free-list for main memory
One bit per block on disk
Bit = 1 , block is free
Easy to find any page free in next 32 bits
Marking block free very simple
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Free-Space Management
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Problems:
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Space: Bitmap has big RAM footprint
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Performance: Slow if most of disk is in use
Alternative: link together free blocks
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80GB disk, 512 byte sectors = 5Mb
Head of list – cached in kernel memory
Each block contains next pointer
Cost of allocation? Freeing? Allocating
consecutive blocks?
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Summary
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Many of concerns & implementations of file
systems similar to virtual memory
implementations
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Contiguous allocation: simple but...
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External fragmentation, compaction, relocation...
Indexed allocation ~ page tables
Free space: managed with bitmap or linked lists
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Next Time
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I/O Systems
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